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if you read the tooltip closely you'll see that casting mind spike on a target will remove all of your DoT effects (VT, SW:P and DP), that means that in a general rotation we never hard cast it. your standard fillers are Mind blast, SW-D (sub-20%) and Mind Flay, in that priority order.
2 situations where you do use mind spike:
a) obviously if you specced into the talent 'from darkness comes light', when this procs, you can cast the instant-cast, free and more powerful mind spike, it will not remove your DoTs
an monster spawns that needs to be killed by your raid/party so quickly that it's not worth putting DoTs on it, on these you can hard cast mind spikes and mind blast. I'd say the limit is a timeframe of about 10 seconds, any DPS time less than this probably won't be worth casting DoTs on.